At the time, federal police believed Duong, who was hospitalised after the explosion, had been trying to kill himself.

"We do believe he knew it was the Australian Christian Lobby, but he was not driven by a motivation or an ideology based on anything the Christian Lobby had done or said," Australian Federal Police Commissioner Andrew Colvin said.

"His motivation was driven by mental illness and his desire to commit suicide."

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ACL director Lyle Shelton said in December he was not aware of the exact motive for the crash, but it did follow "multiple death threats and violence that my staff have endured over the course of this year".

The threats were made over the ACL’s opposition to gay marriage and the Safe Schools program, with one of the threats traced to a person in Tasmania but the rest unable to be tracked, he said.

The ACL office is located less than a 10-minute drive from Parliament House.

- Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14.